______________________________________ References Cited U.S. PATENT DOCUMENTS ______________________________________ 3,645,875 2/1972 Record et al 204/195 4,046,661 9/1977 Stringer et al 204/195 S 4,101,404 7/1978 Blumenthal et al 204/195 S 4,186,072 1/1980 Blumenthal et al 204/195 S 4,193,857 3/1980 Bannister et al 204/195 S 4,588,493 5/1986 Blumenthal et al 204/410 4,814,061 3/1989 Blumenthal et al 204/410 ______________________________________
1. Field of the Invention
The invention is specifically designed to address problems related to measurement and control of carbon potential in furnaces, where special carbonaceous atmospheres are maintained in contact with (usually) ferrous materials at elevated temperatures. Diffusion of carbon into the work pieces is controlled to provide hardening and abrasion resistance in the surface of the part, in a process well known to those versed in the science. The patent describes design factors for improved reliability and maintenance of the zirconia oxygen sensor in this and related fields.
2. Description of Related Art
Since about 1970, rapid commercial development of electrolytic oxygen sensors for carbon potential calculation, has demonstrated the power of this analytical device for controlling furnace atmospheres in most heat treating applications. The electrolyte of choice for this application, is zirconia, partially stabilized with yttria. Sensor construction typically consists of an electrolyte tube, disposed inside a protective sheath which provides protection from abrasion and thermal shock; the assembly is inserted through the furnace wall. The sensing portion is at the very tip of the tube, at the junction of electrolyte, atmosphere and sensing electrode. An inner, positive electrode, bathed in air and in contact with the inner zirconia surface, provides a fixed reference point against which the outer composition is measured. The outer, (negative) electrode completes the electrochemical cell and displays a voltage that is logarithmically related to the furnace atmosphere oxygen composition. Thermodynamic relationships have been derived which provide a predictably precise relationship between the sensor voltage and the carbon potential in conventional furnace atmospheres at temperatures between 1400.degree. F. and 2100.degree. F.
Current commercial versions of the zirconia sensor can be differentiated by the type of outer electrode constructions used for measuring the voltage generated at the sensing surface. The first of these can be described as an "area contact electrode". One of the earliest examples of this type is a sprayed porous layer of refractory chrome nickel alloy applied to the sensing tip of a long (600 mm) zirconia tube and connected to the measuring circuit by a ceramic-protected alloy wire. This electrode proves to be eminently satisfactory in the relatively benign atmospheres from endothermic generators and other low carbon atmospheres at moderate temperatures. It suffers, however, from massive accelerated attack at higher temperatures (above about 1700.degree. F.) and elevated carbon potential levels. Embrittlement, in combination with a mismatch in thermal coefficients of expansion, causes the sprayed alloy electrode to `peel` off the zirconia substrate, resulting in premature failure. Attack by fumes from molten salt quench processing causes dramatic acceleration of this failure. In addition, unusual dynamic behavior has been observed with this construction, and ascribed to "catalytic" activity.
Another example of the area contact electrode consists of direct contact of a flat ended zirconia substrate with, either an alloy mesh or plate, inside the end of the protective alloy sheath. This version displays significant improvement in resistance to extreme conditions. It provides, however, many tiny pockets where carbon can accumulate. This can, in some cases, cause an elevated reading, not typical of the actual furnace atmosphere.
Another common electrode of the area contact type was developed to reduce the cost of the sensing electrolyte, which is a significant part of the sensor price. The substrate manufacturer has tried to address this expense by limiting the amount of zirconia in the sensing element. This is achieved by cementing a small cylindrical plug of zirconia into the end of a long alumina support tube using a "eutectic" ceramic cement. The flat end of this plug is then seated into a small cavity in the tip of the protective sheath, which becomes the sensing electrode. While this provides a functionally satisfactory sensor, the cemented joint is prone to develop cracks, due to temperature cycling to which it is exposed in the typical application. Every sensor of this type can be shown to leak to some extent, although the leakage is acceptably small in `new` sensors. At some stage however, due to temperature cycling, the leak may be so large that it will cause a low measurement of carbon potential, and correspondingly damaging overcarburization. The construction which is intended to reduce the basic cost, may thus create an ultimate loss far greater than any savings realised by the cost savings.
The second type of electrode used in a commercial sensor can be described as a "line contact electrode". An end cap is positioned inside the end of an alloy protective sheath; the cap has a hole with several grooves provided axially in the walls of the hole. A round bottom zirconia tube which has a larger diameter than the hole, is seated in the hole, spring loaded and making interrupted line contact with the edge of the hole. The cap, which is welded into the sheath, is thus the line contact electrode. This type of electrode solves many of the problems listed with the area contact electrodes. It can, however, cause electrolyte fracture due in part to the spring force induced tensile stress concentration at the line of contact with the electrode, and initiated by thermal stress induced by the rapidly changing temperature gradients normally encountered in the heat treating processes, and unrestricted by the large openings in the sheath near the electrode.
An important factor addressed by this patent relates to the openings provided in the protective sheath of commercial probes to facilitate contact with the furnace atmosphere. These have, in some designs, been made both numerous, and large, in the interest of promoting good contact of the atmosphere with the sensing surface, hence, rapid response to changes in the atmosphere. Experience has shown that diffusion rates are proportional to the absolute temperature, and that not only is such openness not essential to fast response, it doesn't impede exposure to temperature effects, and can cause some totally unexpected problems, related to effective maintenance. The techniques we have pioneered over the past twenty years for maintaining a carbon sensor free of the damaging effects of soot involve two concurrent operations: 1) air is pumped into the probe annulus to oxidize the fluffy carbon deposited therein and 2) the electrodes are shorted out to cause oxygen ions to flow through the zirconia substrate and emerge as nacent oxygen, which will consume the more tenacious carbon deposited on the sensing surface. The large sheath openings in some current commercial devices, require excessive air to be pumped into the annulus in order to provide adequate oxygen for combustion of the soot. The carbon potential in the furnace can accordingly drop significantly, resulting in parts decarburization.